Although adults with Down syndrome (DS) are believed to be at increased risk for dementia of the Alzheimer's type, current evidence does not provide a clear picture of the natural history of dementia in this population. Evidence is particularly lacking regarding the early stages of dementia and its similarities to dementia in persons without mental retardation (MR). Early diagnosis and knowledge about similarities to dementia in the general population are important to efforts to identify the etiology of dementia and to determine appropriate intervention. The diagnosis of dementia in adults with DS is complicated, however, because there are no well-accepted diagnostic methods, and those used in adults without MR are not often appropriate. In adults with MR, diagnosis is further complicated by a lack of knowledge about the effects of normal aging, health status, psychiatric disorders, and drug use on functioning. These problems limit our current ability to understand the relationship between DS and Alzheimers Disease. The proposed study will increase understanding of the natural history of dementia in adults with DS and other forms of MR by providing a prospective comparison of their neuropsychological and behavioral functioning. Adults aged 30 to 70 (N= 170) will be seen for 4 yearly assessments of neuropsychological functioning, psychopathology, and health status. The first aim of the study is to determine changes in functioning related to normal aging in adults with DS and in an age matched group of MR adults without DS. Changes in functioning indicative of dementia will then be operationally defined as significant deviations from normal. The second aim is to examine relationships between dementia and other conditions that can contribute to or mimic dementia (e.g., illness, psychiatric disorders, drug use). The third aim is to compare clinical judgment to objectively.derived criteria for the detection of dementia in adults with MR. Results will contribute to improved diagnosis, treatment and prediction of risk for dementia.